Most vehicles in operation today (and many other devices) are powered by internal combustion (IC) engines. An internal combustion engine typically has a reciprocating piston which oscillates within a cylinder. Combustion occurs within the cylinder and the resulting torque is transferred by the piston through a connecting rod to a crankshaft. For a four-stroke engine, air, and in some cases fuel, is inducted to the cylinder through an intake valve and exhaust combustion gases are expelled through an exhaust valve. In typical engine operation, the cylinder conditions vary in a cyclic manner, encountering, in order, an intake, compression, expansion, and exhaust stroke in a repeating pattern. Each repeating pattern may be referred to as a working cycle of the cylinder.
Internal combustion engines typically have a plurality of cylinders or other working chambers in which an air-fuel mixture is combusted. The working cycles associated with the various engine cylinders are temporally interleaved, so that the expansion stroke associated with the various cylinders is approximately equally spaced, delivering the smoothest engine operation. Combustion occurring in the expansion stroke generates the desired torque as well as various exhaust gases. The expansion stroke is often denoted as the combustion or power stroke, since this is the power producing stroke.
Under normal driving conditions, the torque generated by an internal combustion engine needs to vary over a wide range in order to meet the operational demands of the driver. Over the years, a number of methods of controlling internal combustion engine torque have been proposed and utilized. Some such approaches contemplate varying the effective displacement of the engine. Two different engine control approaches that vary the effective displacement of an engine include: (1) the use of multiple fixed displacements; and (2) skip fire engine operation. In fixed multiple displacement control some fixed set of cylinders is deactivated under low load conditions; for example, an 8-cylinder engine that can operate on the same 4 cylinders under certain conditions. In contrast, skip fire control operates by sometimes skipping and sometimes firing a cylinder. In some engines all cylinders are capable of firing or skipping, while in other engines only a subset of the engine's cylinders have skip fire capability. In general, skip fire engine control is understood to offer a number of potential advantages, including the potential of significantly improved fuel economy in many applications. Although the concept of skip fire engine control has been around for many years, and its benefits are understood, skip fire engine control has only recently obtained some commercial success.
It is well understood that operating engines tend to be the source of significant noise and vibrations, which are often collectively referred to in the field as NVH (noise, vibration and harshness). In general, a stereotype associated with skip fire engine control is that skip fire operation of an engine will make the engine run significantly rougher, that is with increased NVH, relative to a conventionally operated engine. In many applications, such as automotive applications, one of the most significant challenges presented by skip fire engine control is vibration control. Indeed, the inability to satisfactorily address NVH concerns is believed to be one of the primary obstacles that has prevented widespread adoption of skip fire types of engine control.
U.S. Pat. Nos. 7,954,474, 7,886,715, 7,849,835, 7,577,511, 8,099,224, 8,131,445, 8,131,447, 8,616,181, 8,701,628, 9,086,020 9,328,672, 9,387,849, 9,399,964, 9,512,794, 9,745,905, and others, describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a skip fire operational mode. Each of these patents and patent applications is incorporated herein by reference. Although the described controllers work well, there are continuing efforts to further improve the performance of these and other skip fire engine controllers to further mitigate NVH issues and improve fuel economy in engines operating under skip fire control. The present application describes additional skip fire control features and enhancements that can improve engine performance in a variety of applications.